Developers used in electrophotographic systems such as copying machine include two-component developer, which comprises carrier and toner, and single component developer, which contains no carrier. Conventional toners used in both developers are fine particles of binder resin in which colorants, such as pigment and dye, are dispersed.
One of the important properties of the toners is triboelectric chargeability. Properties required for toners include that they can generate a suitable amount of positive or negative electrostatic charge upon contacting with the carrier or charge providing device, and the amount of the charge is stable during continuous use or under an adverse environment. Binder resin, the main component of the toner may be triboelectrically chargeable but the amount of the generated electrostatic charge is not sufficient. When an image is developed with a toner containing no charge control agent, image fogging may occur to make the resulting image unclear. In order to impart further triboelectric charging property, toners in general are added with a charge control agent so that the electrostatic charge is controlled suitably.
Generally known and used charge control agent for electrophotographic toners include positively charging agents such as nigrosin dye and quaternary ammonium salt, and negatively charging agents such as metal containing monoazo dyes, metal-salicylic acid complex and copper phthalocyanine pigment.
Problems to be Solved by the Invention
Although known charge control agent, especially negative charge control agent has an excellent chargeability, they are not suitable for manufacturing color toner because of their color attributable to their main component, pigments or dyes.
Metal complex charge control agent has poor dispersibility and compatibility with the resin and may cause inhomogeneous charge of the resulting toner. Further, the agent may easily be released from the toner particles to stain on the development sleeve or carriers or lower the chargeability over continuous copying. Said problems may cause reduction of resulting image density.
Further, the complex structure of a metal containing compound like metal complex invites high manufacturing cost. The metals like chrome may affect negatively on human body and environment. There has been a demand for development of metal free charge control agent.
Japanese Patent Laid Open No. H05-11505 discloses to use a compound obtainable by linking 3-carboxy substituted 2-naphthols with alkylene such as pamo acid as a charge control agent.
The alkylenebisnaphthol derivative disclosed in this prior art has some triboelectric charging ability but the chargeable amount is low. Therefore, said derivatives cannot be used as a charge control agent for actual electrophotographic system.
In order to dissolve the above problems, an object of the present invention is to provide a novel alkylenebisnaphthol derivative which is suitably used as charge control agent for electrophotographic toner and the like.
Further object of the present invention is to provide a novel charge control agent having an excellent dispersibility and compatibility with binder resins and is suitable for manufacturing color toners, and is safe for human body as well as environment.
Still further object of the present invention is to provide electrophotographic toner of which particles can be charged uniformly within a short time period and the triboelectric charging property is stable over time. Said toner can provide high quality images with high image density and less fogging.
Solution of the Problem
The inventors have successfully synthesized novel alkylenebisnaphthol derivatives, studied their physical properties and found that the derivatives have an excellent triboelectric charging property and are useful as charge control agent for electrophotographic toners.
Accordingly, the present invention provides an alkylenebisnaphthol derivative represented by the formula [I]:
wherein Y1, Y2, Y1′ and Y2′ may be same or different and each of them is selected from the group consisting of carboxyl group, an esterified carboxyl group, a group of —(CONH)n-X (wherein X is an optionally branched and optionally substituted hydrocarbon group which may have an unsaturated bond, an optionally substituted aromatic group and a heterocyclic group having conjugated double bonds, n is an integer of 1 or 2);
k is an integer of 1–12;
R and R′ are selected from the group consisting of hydrogen atom, an alkaline metal, an optionally branched and optionally substituted alkyl or acyl group of 1–6 carbon atoms and a phenylalkyl group;
Q and Q′ are selected from the group consisting of an optionally branched alkyl or alkoxy group of 1–6 carbon atoms, a halogen atom, nitro group, nitroso group, amino group and sulfo group; and
m and m′ each represents an integer of 0–3 or a salt thereof.
The present invention further provides charge control agent consists of the above alkylenebisnaphthol derivative or its salt, and an electrophotographic toner comprising said compound as a charge control agent. Further more, the present invention provides use of the above-identified alkylenebisnaphthol derivative or a salt thereof as a charge control agent.
In the above formula, examples of esterified carboxylic groups for Y1, Y2, Y1 and Y2′ include an alkoxycarbonyl of 1–6 carbon atoms, such as methoxycarbonyl, ethoxycarbonyl, propyloxycarbonyl, isopropyloxycarbonyl, butoxycarbonyl, pentyloxycarbonyl and hexyloxycarbonyl; phenoxycarbonyl and phenacyloxycarbonyl. In case the group has an aromatic moiety, said moiety may have a substituent.
The group X may be an optionally branched and optionally substituted hydrocarbon group which may have an unsaturated bond, preferably an alkyl of 1–20 carbon atoms such as methyl, ethyl, propyl, butyl, pentyl, hexyl, decyl, dodecyl, lauryl and stearyl; an alkenyl of 2–6 carbon atoms such as vinyl, allyl, propylenyl, butylenyl, pentylenyl and hexylenyl; an optionally substituted aromatic group such as, phenyl, naphthyl, anthryl, anthraquinonyl and pyrenyl; and an optionally substituted heterocyclic group having conjugated double bonds such as benzimidazolonyl, carbazolyl, pyridyl, thiazolyl, benzothiazolyl, imidazolyl, indryl, thiofuryl, phenothiazinyl, acridinyl and quinolinyl.
Examples of substituents in each definition, wherein the group is optionally substituted, may include a halogen atom, a halogenated lower alkyl, nitro, a lower alkyl, a lower alkoxy such as methoxy, cyano, phenyl, naphthyl, phenoxy, furyl, amino, toluidylamino, triazylamino, pyrimidylamino, benzoylamino, sulfo, hydroxy, an esterified carboxyl group such as alkoxycarbonyl and phenoxycarbonyl, amidized carboxyl groups such as phenylaminocarbonyl, alkylaminosulfonyl group and an alkenyl group of 2–6 carbon atoms which may include aryl group.
When the substituent contains an aromatic ring, the compound may further have one or more substituents such as a halogen atom, a lower alkyl, a lower alkoxy, phenyl, and cyano groups on said aromatic ring.